1. Field of the Invention
The present invention relates to methods of and apparatuses for inspecting printed circuit boards or substrates (referred to as substrates in this specification) used for inspecting parts mounted on the substrate for acceptability of soldering.
2. Description of the Related Art
Parts mounted on the surface of a substrate have been heretofore visually inspected for acceptability of their mounted state. In particular, the presence or absence, the amount, the solubility, the short, the inferior conduction, and the like of solder are visually inspected, to determine whether or not their soldered state is good. In such visual inspection, however, occurrence of inspection errors is not avoided, the results of the determination vary depending on inspectors, and there is a limitation to the inspection processing capability.
In recent years, various automatic inspecting apparatuses capable of automatically making this type of inspection have been proposed.
The surface of a soldered portion has a shape which extends in three dimensions. In order to inspect the shape, it is essential that information on three-dimensional shapes can be detected.
FIG. 1 shows an example of an automatic inspecting apparatus capable of inspecting information on three-dimensional shapes, which projects slit light 1 to a soldered portion on a substrate 2. Reflected light of a light cutting line 3 formed on the surface of the substrate 2 including the soldered portion by projecting the slit light 1 is imaged by an imaging unit 4. Its imaged pattern is examined, thereby to detect the three-dimensional shape of the soldered portion.
In this inspecting method, however, information on the shape of a portion illuminated by the slit light 1 is only obtained. Accordingly, it is difficult to grasp the three-dimensional shapes of other portions.
In order to solve this problem, there is provided a method of projecting light to the surface of a solid bounded by curved surface which is an object to be inspected from a plurality of directions at different angles of incidence, imaging its respective reflected light from the surface of the solid bounded by curved surface, and detecting the orientation of the element constituting the curved surface of the solid bounded by curved surface from their respective imaged patterns. This method belongs to an "active sensing method" which is one of methods of detecting information on three-dimensional images. More specifically, this method pays attention to the fact that, when a light beam having a constant pattern is projected to an object to be inspected, the pattern of its reflected light beam obtained from the object to be inspected is deformed corresponding to the three-dimensional shape of the object to be inspected, to estimate the shape of the object to be inspected from the deformed pattern.
FIG. 2 is a diagram for explaining the principle of this method, showing the positional relation between a detecting system comprising a light projecting unit 5 and an imaging unit 6 and a solid bounded by curved surface which is an object to be inspected.
It is assumed that, when a light beam 8 is projected to the surface of a solid bounded by curved surface for example, a soldered portion) 7 from the light projecting unit 5 arranged in a given position, its reflected light beam 9 is incident on the imaging unit 6 placed directly over the solid 7, to be detected. In this case, it is determined that the element of the curved surface of a portion illuminated by the light beam 8 to the solid bounded by curved surface 7 is oriented at an angle of .theta. with a horizontal reference surface 10 (.theta. is the angle of incidence). Accordingly, in a case where the nature of the surface of the solid bounded by curved surface 7 comprises a lot of elements of the curved surface oriented in different directions like the surface of the soldered portion, if light is projected to the surface of the solid 7 using a plurality of light projecting units having different angles of incidence, a group of elements of the curved surface corresponding to the respective angles of incidence is detected by the imaging unit 6. Consequently, it can be determined how each of the elements of the curved surface of the solid bounded by curved surface 7 is oriented, that is, how the nature of the surface of the soldered portion is.
Furthermore, if the light projecting unit 5 projects the light beam 8 having a width of .DELTA..theta., the reflected light beam 9 having a width corresponding to the width is detected by the imaging unit 6. More specifically, in this case, the element of the curved surface having an angle of .DELTA..theta. can be detected.
Additionally, if the light projecting unit 5 includes ring-shaped light sources 11, 12, 13 located horizontally relative to the reference surface 10, the distance between the light projecting unit 5 and the solid bounded by curved surface 7 is constant even if the solid 7 has any angle of rotation with an axis perpendicular to the reference surface 10. Accordingly, the orientation in the direction of the angle of rotation of the element of the curved surface is canceled. Consequently, only the angle of inclination of the solid bounded by curved surface 7 to the reference surface 10 is detected.
Furthermore, as shown in FIG. 3, if the light projecting unit 5 comprises a plurality of ring-shaped light sources 11, 12 and 13 having different angles of incidence to the solid bounded by curved body 7, the elements of the curved surface having orientations corresponding to the angles of incidence of light beams 14, 15 and 16 from the sources 11, 12 and 1 can be specifically detected as described above.
The three ring-shaped light sources 11, 12 and 13 having radii of r.sub.m (m=1, 2, 3) are horizontally arranged in positions at the heights r.sub.m (m=1, 2, 3) from a referenCe surface 10. In addition, let .theta..sub.m (m=1, 2, 3) be the angles of incidence of the light beams 14, 15 and 16 from the light sources 11, 12 and 13 to the solid bounded by curved surface 7. In this case, the elements of the curved surface respectively having angles of inclination of .theta..sub.m in the solid bounded by curved surface 7 can be detected by the imaging unit 6. The size of the element of the curved surface is sufficiently smaller than the total optical path length leading to the imaging unit 6 from the light sources 11, 12 and 13 through the surface of the solid bounded by curved surface 7. Consequently, the angle of incidence, that is, the angle of inclination of the element of the curved surface to be detected can be set by the following equation: EQU cos .theta..sub.m =h.sub.m /(h.sub.m.sup.2 +r.sub.m.sup.2).sup.1/2( 1)
As a method of checking the appearance of a soldered portion on the basis of the above described principle, a method using white light sources as the light sources 11, 12 and 13 has been proposed (Japanese Patent Application laid-open Publication No. 61-293657). In this checking method, the three light sources 11, 12 and 13 having different angles of incidence to a soldered surface are respectively turned on or off at different timings so as to mutually identify images formed by reflected light from the light sources 11, 12 and 13.
However, in such a lighting control method, a memory for storing images obtained at different timings of projecting light, an arithmetic unit for executing an arithmetic operation taking the images as the same field image, a lightning unit for causing each of the light sources to instantaneously perform a lightning operation, and the like are required. Accordingly, the method is complicated from the technical standpoint, which presents problems in terms of the cost and the reliability.
In order to solve the problems in such a time sharing method once and for all, the applicant has proposed the following substrate inspecting apparatus (see Japanese Patent Application No. 63-112054).
This substrate inspecting apparatus is characterized by comprising light projecting means including three types of ring-shaped light sources for respectively producing such red, green and blue light, each having a light energy distribution using the wavelength to enter the horizontal axis, as to be white light by mixing and arranged in positions where the light are emitted to the surface of a solid bounded by curved surface to be inspected obliquely from above at different angles of inclination, light amount adjusting means for adjusting the amount of light of each of the light sources such that the light emitted from the light sources are mixed to be white light, imaging means for imaging reflected light from the surface of the solid bounded by curved surface to be inspected directly over the solid to obtain imaged patterns by hues (for each color), and processing means for detecting the nature of the element of the curved surface of the above solid bounded by curved surface from the imaged patterns obtained by the above imaging means.
The red light, the green light and the blue light from the ring-shaped light sources are directed to the surface of the solid bounded by curved surface to be inspected at different angles of incidence. The substrate inspecting apparatus is constructed such that images formed by the red, green and blue reflected light can be simultaneously detected from the surface of the solid bounded by curved surface by hues. Consequently, the nature of the solid bounded by curved surface can be detected using the pattern of the hue obtained in a short time. In addition, the red light, the green light and the blue light emitted from the light sources are mixed to be white light. Accordingly, peripheral information indispensable in inspection of a soldered state of mounted parts, for example, information on parts on a substrate (the number, the polarity, the color code or the like of the part) and information on substrate patterns (various marks or the like) can be detected. Thus, the soldered state can be automatically inspected.
However, in such a substrate inspecting apparatus, if the surface of a soldered portion has a gentle slope, its reflected light is introduced into imaging means. On the other hand, if the surface of the soldered portion has a steep slope (at an angle of 45.degree. or more with the surface of the substrate), its reflected light is not incident on the imaging means. Consequently, it becomes difficult to inspect the nature of the soldered portion.